🧊 Hey There — From Mark’s Workshop to Yours
Hey hey — Mark here. 👋 If you’re reading this, you probably know one thing for sure: summer doesn’t wait. That means if you want to keep your house, your basement, or your bonus room comfortable, you need a through-the-wall AC that’s properly sized — or you’ll end up with a unit that’s short-cycling and wasting energy or one that can barely keep up.
And believe me, I’ve seen my fair share of botched installations and half-baked guesses. Nothing’s sadder than hearing someone say they bought “whatever was on sale” only to realize their new AC is a runt that can’t chill the room — or a behemoth that freezes the room too fast and then shuts off before dehumidifying.
That’s why I decided to whip up this playbook — to help you nail the sizing part like a pro. Getting the BTUs and all the other sizing factors right will make sure you’re not blowing money on an AC that’s either too big or too small. You’ll end up with a comfortable, even temp that lets you sleep at night and smile when you see your utility bill.
In this deep-dive, we’ll cover:
✅ What BTUs mean and why they matter
✅ How to measure your space like a pro
✅ Factors that change the “standard” BTU number — ceiling height, sun exposure, insulation, and more
✅ Common mistakes and pitfalls to dodge
✅ Resources and tools that make sizing easier — including a few blue-clickable links so you can do your homework
So if you’re ready to crush this process with the precision of a veteran tech, let’s jump in. 🔧❄️
❄️ 1️⃣ Understanding BTUs: The Heart of Sizing
When you shop for a through-the-wall AC, you’ll see a number attached — usually something like 8,000 BTUs, 12,000 BTUs, or 18,000 BTUs. That number is the British Thermal Units per hour — essentially a measure of how much heat the unit can remove from the room.
More BTUs = more power. But bigger isn’t always better. Oversized ACs cycle too fast and leave humidity behind. Undersized ACs struggle and burn up energy without ever making you comfy.
A good rule of thumb? Aim for about 20 BTUs per square foot of your space. For example, a 400-square-foot room will need around 8,000 BTUs. This is a starting point, but — as you’ll see — there’s more to the story.
💡 Pro tip: If you want a simple calculator to check your math as you go, the HVAC Ninja BTU calculator is a solid place to begin.
❄️ 2️⃣ Measuring Your Space Properly
Before you pick an AC off the shelf, break out the measuring tape. Multiply the length and width of your room to get the square footage. That’s your baseline BTU number — but then we need to adjust for the things that make your space a bit cooler or hotter than average.
Here’s a quick cheat sheet:
-
Ceilings higher than 8 feet? Multiply BTUs by 1.1 for every extra foot of height.
-
Heavy afternoon sun through windows? Bump up BTUs by 10%–20%.
-
Excellent insulation? Drop BTUs by 10%–15%.
-
Kitchen or laundry? Add about 4,000 BTUs to compensate for appliance heat.
-
More than 2 people regularly using the room? Add 600 BTUs per extra person.
That might sound like a lot of math, but it’s really just fine-tuning. Skip this part, and you risk buying an underpowered unit that can’t keep up.
And remember — getting this right also pays off long-term. Check out the Department of Energy’s AC Sizing Guidelines for a deeper dive into what the pros look for.
❄️ 3️⃣ Common Mistakes to Avoid
Over the years, I’ve seen too many folks make these sizing blunders:
-
Ignoring Insulation: Poorly insulated walls, leaky windows, or cracks around doors mean extra heat sneaks in. Seal these up before buying your AC so you don’t need a bigger, more expensive unit.
-
Using Guesswork Instead of Measuring: “This one looks fine” is a risky way to choose an appliance that you’ll live with for years. Take the time to measure, calculate, and check your work.
-
Overcompensating with a Giant AC: Bigger is not better — too powerful a unit will short-cycle and leave the room clammy.
-
Ignoring Window Placement and Shade: Rooms with direct sunlight need more power; shaded basements need less.
-
Overlooking Heat-Generating Devices: Big-screen TVs, gaming rigs, and even houseplants give off heat. Factor that into your BTU number too.
For more pointers and pro tricks, you might enjoy this Trane article on sizing ACs properly, which lays out the pitfalls most homeowners face.
❄️ 4️⃣ The Benefits of Getting It Right
Why go to all this trouble? Because sizing your through-the-wall AC properly pays you back big time:
✅ More Consistent Comfort — No “hot spots” or noisy short-cycling.
✅ Better Energy Efficiency — The right size AC uses less electricity, saving you money.
✅ Longer Lifespan — Less strain on the compressor and fan means your unit can last 10–15 years.
✅ Humidity Control — Proper cycle time means drier, more comfortable air.
✅ Less Noise & Wear — Right-sized units don’t work as hard, so they’re quieter and less prone to breakdowns.
And if you’d like a quick reference to compare different BTU ratings and typical room sizes, The Furnace Outlet’s through-the-wall AC sizing chart is super helpful to bookmark.
❄️ 5️⃣ Seasonal and Environmental Factors
One thing I often see overlooked is seasonal variation and your local climate. If you live in a humid, tropical area, you may need to scale up BTUs. In drier climates, you can probably scale down slightly.
You can also look into units with heat pumps for year-round use — especially if you’re somewhere like the Northeast where winters bite. That way your through-the-wall unit keeps you warm as well as cool, making it a true all-in-one HVAC solution.
And don’t underestimate the power of shading, curtains, and landscaping. Strategic tree placement or awnings can reduce the heat gain by up to 77%, according to the U.S. Department of Energy’s cooling efficiency article.
❄️ 6️⃣ Tools and Resources to Help You Size
Don’t want to do the math manually? I get it — and you don’t have to.
✅ Check out AHRI’s AC Sizing Guidelines for quick overviews.
✅ Look into the BTU calculators built into most top HVAC websites.
✅ Have a pro come do a heat load calculation if you want extra assurance.
And remember — whatever calculator you use, take your lifestyle into account. If you cook a ton, hang laundry indoors, or run computers all day in that space, you’ll appreciate a bit of extra capacity built into your BTU number.
❄️ 7️⃣ Why Through-the-Wall Units Are Worth the Extra Thought
Before I sign off, let’s remember why we care about all this: through-the-wall ACs aren’t just an appliance; they’re a semi-permanent fixture that you’ll live with for a long time. Installing one is a bit more involved than popping a window unit in, so you want to make sure you nail this sizing part up front.
When you size properly, you get all the perks — stable temperatures, solid energy savings, less maintenance — and none of the headaches. Trust me, it’s worth taking the extra steps.
🧊 Outro — From My Workshop to Yours
Whew — that was a deep dive, but you know what? That’s exactly what this kind of project deserves. 🧰 From measuring your room like a pro to tweaking your BTU number for every quirk of your space, proper sizing is the secret sauce that turns a plain through-the-wall AC into a long-term comfort machine.
And if you ever need to double-check those numbers, check the links sprinkled above — especially The Furnace Outlet’s through-the-wall ACs page, where you can match the perfect unit to your calculations.
Want some electrical and wiring tips for your wall AC? Visit my guide Wiring Wisdom: Mastering the Electrical Essentials for Through-the-Wall ACs.
As always, I appreciate you sticking around and geeking out with me. Share this guide if you know someone tackling their own AC upgrade — because at the end of the day, a comfy home is a happy home.
That’s my cue to call it a day. Until next time, stay cool, stay handy, and keep making your home the most comfortable place to kick back. You can do this — and I’ll be right here if you ever want me to talk you through the next big project. ❄️💪
— Mark, your go-to HVAC guy!
